One of our justifications for the Monarch Waystation program has been the loss of milkweeds in 100 million acres of row crops* that followed the adoption of Roundup Ready soybeans and corn. In June of 2004 I wrote “Effects of Transgenic Crops on Milkweeds” where I drew attention to this issue and to the rapid development of weeds resistant to Roundup (glyphosate) in Argentina and concluded “Roundup resistant weeds seem to be in our future.” They are here and are spreading rapidly.

When I wrote the June 2004 article, 15 weed species in Argentina were notably resistant to Roundup. According to Dan Charles’s report there are already 13 Roundup resistant species in the United States. We are catching up fast.

Curiously, the use of Roundup is changing the composition of weed communities due to selection that favors species that are naturally resistant to Roundup such as lambsquarters, velvetleaf, giant ragweed, and Asiatic dayflower. Other species, with genetic variants that are resistant to Roundup, are increasing as well. This scenario may apply to wild parsnip, pokeweed, evening primrose, and horseweed – species that are showing up in row crops, a habitat in which they had previously been relatively uncommon.

*Note: Genetically modified seeds for major crops are now the rule, not the exception. In 2007, 92 million acres of corn were planted and of these 61% had been genetically engineered to express Bt toxins to kill or inhibit several types of insects, to resist Roundup, or both. Of the 64 million acres of soybeans planted in 2007, 91% were Roundup Ready. Due to the soy/corn rotation, my interpretation is that we’ve lost at least 100 million acres of milkweeds in these row crops since the first Roundup Ready plants were brought into production in 1996. This is an increase from my earlier estimate of a loss of milkweeds in 80 million acres of row crops. If you find milkweeds in corn or soybean fields, take a picture – it’s now a rare sight.